Typical examples of the tractor transmission assembly of the type set forth above are disclosed in U.S. Pat. No. 5,058,455 and in U.S. Pat. No. 5,058,459. In each of the transmission assemblies disclosed in these U.S. patents, the front housing is fashioned to have an open rear end and the transmission casing is fashioned to have an open front end. A bearing support frame which is fixedly supported by the transmission casing is disposed within a rear end portion of the front housing and is used for supporting shafts of an additional speed change mechanism within the front housing and for supporting shafts of a primary speed change mechanism within the transmission casing. More specifically, each of the above-referenced transmission assemblies comprises a primary shaft which extends through an internal partition wall of the front housing. The additional speed change mechanism includes, other than a rotatable output member (a rotatable output shaft or an output gear) which is driven to rotate by the primary shaft at a variable speed of rotation, a countershaft which extends parallel to the primary shaft, whereas the primary speed change mechanism is fashioned such that it performs a speed change transmission between a drive shaft, which is driven to rotate by the above-referenced rotatable output member, and a speed change shaft which extends parallel to the drive shaft. In the transmission assembly disclosed in U.S. Pat. No. 5,058,455, the above-referenced bearing support frame is used such that it supports rear ends of the primary shaft and countershaft, which are supported at their front portions by the internal partition wall, and for supporting front ends of the drive shaft and speed change shaft. In the transmission assembly disclosed in U.S. Pat. No. 5,058,459, the bearing support frame has a frame structure composed of axially spaced integral front and rear plate-shaped portions. The front plate-shaped portion is used for supporting the primary shaft and countershaft at their front portions, whereas the rear plate-shaped portion is used for supporting a rear end of the countershaft and for supporting front ends of the drive shaft and speed change shaft.
It is intended that the additional speed change mechanism disclosed in each of the above-referenced U.S. patents is preassembled in front of the bearing support frame, which is fixedly supported by the transmission casing, in a condition such that the bearing support frame and the change mechanism supported by this frame project forwardly of the transmission casing. The preassembled additional change mechanism is then insertingly assembled into the front housing such that front portions of shafts of this change mechanism are supported as required. Such a final assembling operation of the additional change mechanism into the front housing is not necessarily carried out easily because of the required positioning of shafts. In the transmission assembly disclosed in U.S. Pat. No. 5,058,455, the preassembled additional change mechanism must be inserted into the front housing by a distance corresponding to the axial width of this change mechanism, and front portions of the primary shaft and countershaft must be positioned and supported at a location spaced largely from the rear end opening of the front housing. The bearing support frame, which is adapted to support shafts of both of the additional change mechanism and primary change mechanism, has front and rear shaft-supporting portions and, therefore, a large axial width or thickness so that the whole of the additional change mechanism inclusive the support frame is large in its axial width. In the transmission assembly disclosed in U.S. Pat. No. 5,058,459, the additional change mechanism is preassembled in front of the bearing support frame in a condition where the primary shaft has not been incorporated in this preassembled change mechanism yet. The primary shaft of a large length is then inserted through an internal partition wall of the front housing and through a shaft-supporting portion in the bearing support frame. Such an insertion of the lengthy primary shaft will make it difficult to position and support this shaft so that the final assembling operation cannot be carried out easily.
The bearing support frame employed in each of these transmission assemblies has a relatively large size and a complicated configuration. Such a bearing support frame is unfavorable for manufacturing and for compactness of the transmission. The bearing support frame shown in U.S. Pat. No. 5,058,459, which comprises front and rear plate-shaped portions, requires that components of the additional change mechanism, such as gears and the like, be placed in positions from a lateral side of the support frame so that assembling operation of the change mechanism is troublesome and time-consuming. In addition, the internal partition wall of the front housing is not used as a positive support member for the additional change mechanism.
Accordingly, a primary object of the present invention is to provide a novel transmission assembly for tractors which facilitates the assembling operation thereof, particularly a final assembling operation in which shafts are positioned and supported.
An attendant object of the invention is to provide a novel transmission assembly in which a bearing support frame disposed within a rear end portion of a tractor front housing is simple in configuration and small in size so that it reduces cost and contributes toward compactness of the transmission.
Another attendant object of the invention is to provide a novel transmission assembly in which components are arranged so as to highten the compactness of the transmission.
A further attendant object of the invention is to provide a novel transmission assembly in which the number of components is minimized so as to reduce cost.